Sand exclusion liners (also known as "sand control screens") are commonly used in wells producing from a sand formation or reservoir. These wells usually are completed "open hole". That is, the wellbore is drilled vertically down through the overburden to the top of the sand reservoir and cased. The end plug of the casing is then drilled out and the wellbore is whipstocked and drilled horizontally through the reservoir. The horizontal section of the wellbore is left uncased or "open hole". A perforated liner is then run into the well at the end of a pipe string and landed or positioned to extend from the end of casing through the horizontal section of the wellbore. The liner is sealed at its inner end to the casing's bottom end, so that the former is an extension of the latter. A production tubing string is then run into the well to extend into the liner bore.
The function of the liner is to allow produced fluid to enter the production string while simultaneously screening or rejecting mobilized sand grains seeking to enter with the fluid.
One specific, commercially available sand exclusion liner will now be described, by way of example. The liner is formed of tubular steel joints coupled together end to end by threaded couplings. Some of the joints have screen sections. The screened joints each comprise a base pipe having transverse perforations extending through the pipe side wall. Steel rods extend across the perforated section, parallel to the axis of the pipe. The rods are welded to the outside surface of the base pipe at spaced intervals around its circumference. A stack of vertically spaced apart steel rings is positioned over the rods; the rings are individually welded to the rods. The rings each have a generally triangular cross-section so that the slot, formed between a pair of adjacent, vertically spaced apart rings, has increasing width from the outside in. This is commonly referred as a "keyhole" opening or slot. In summary, the liner comprises one or more joints, some blank and some having a perforated base pipe carrying means for screening sand to prevent it entering the perforations.
The present invention was developed in connection with experimental wells in which liners were to be landed in horizontal wellbores completed in an unconsolidated sand reservoir.
Two problems needed to be addressed.
Firstly, the liner has a tendency to pile up sand ahead of it as it is advanced along the horizontal wellbore. Or alternatively sand may slump in around the liner. In both cases, it can become difficult or impossible to keep advancing the liner to get it fully inserted to the desired landing position. It is common oilfield practice to remove sand blockages by circulating oil or drilling fluid down through a non-perforated tubular string and out of the annulus or vice versa, to fluidize and remove the sand. However, in the case of a perforated liner, the circulating fluid will reverse prematurely through the perforations and not reach the end of the liner. Thus, circulating fluid to wash out sand blockages preventing full insertion of the liner in a horizontal wellbore is not available as a solution.
Secondly, it will be desirable in some cases to be able to keep the liner and the positioning string to which it is attached empty or "dry " when easing it into the horizontal section of the wellbore. If the liner/string unit is filled with the fluid filling the wellbore, then the liner will bear against the bottom surface of the horizontal wellbore section wall. Being heavy and with little string weight to help force it along, the liner is difficult to advance. If the liner/string unit can be kept empty, then buoyancy will assist in easing the liner along the horizontal wellbore.
With this background in mind, it was therefore one objective of the invention to modify a liner so that it could be "circulated into" the wellbore. It was a second preferred objective to provide a liner that could be "floated dry" into the wellbore.